The present invention relates to a polymer inhibiting composition and to a process for inhibiting the polymerization of a readily polymerizable vinyl aromatic compound.
It is well known that vinyl aromatic compounds such as monomeric styrene, lower alkylated styrene such as alpha-methylstyrene and the like, polymerize readily and furthermore, that the rate of polymerization increases with increasing temperature. Inasmuch as vinyl aromatic compounds produced by common industrial methods contain impurities, these compounds must be subjected to separation and purification processes in order to be suitable for most types of further industrial use. Such separation and purification are generally accomplished by distillation.
In order to prevent polymerization during storage of vinyl aromatic compounds, various types of known polymerization inhibitors have been employed usually under refrigerated conditions. For example, common inhibitors useful for inhibiting the polymerization of vinyl aromatic compounds during storage conditions include 4-tert-butylcatechol (TBC) and hydroquinone.
Sulfur, on the other hand, has been widely employed as a polymerization inhibitor during the distillation of various vinyl aromatic compounds. However, while sulfur provides a reasonably effective inhibitor, its use in such distillation processes results in a highly significant disadvantage namely, there is formed in the reboiler bottom of the distillation column a valueless waste material highly contaminated with sulfur. This waste material, furthermore represents a significant problem of pollution and/or waste removal.
Although many compounds are effective for inhibiting the polymerization of vinyl aromatic compounds under differing conditions such as storage, only some of these compounds have proved to be of any real utility for inhibiting vinyl aromatic polymerization under distillation conditions. One compound found effective for polymerization inhibition is 2, 6-dinitro-p-cresol (DNPC) disclosed in U.S. Pat. No. 4,105,506 by Watson. In addition, it has been found that previously known polymerization inhibitors may be combined to achieve an inhibitory effect greater than either inhibitor alone. The synergistic effect of combining two known inhibitors was disclosed in U.S. Pat. No. 4,061,545 by Watson, wherein phenothiazine and tert-butylcatechol (TBC) were used together in the presence of oxygen as a polymerization inhibitor. The synergistic effect of N-nitrosodiphenylamine combined with DNPC in inhibiting the polymerization of vinyl toluene under vacuum conditions was disclosed in U.S. Pat. No. 4,341,600 by Watson. It has been found, however, that as the distillation temperature increases, the effectiveness of these inhibitors decreases.
During distillation of vinyl aromatic compounds, higher distillation temperatures are preferred in a distillation apparatus in order to achieve a higher throughput and a more energy efficient distillation. These higher temperatures, however, also result in an increased rate of polymerization which leads to unacceptable levels of polymer in the distillation apparatus. Accordingly, therefore, there exists a strong need for a polymerization inhibitor which will effectively prevent the polymerization of vinyl aromatic compounds during distillation at higher temperatures.